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32
THE SCIENTIFIC STORY
ji
dence to subsequent, less specific tests such as the
iodide method outlined above.
Another analytical method is based on the fact
that ozone is capable of cracking bent rubber
strips in a characteristic fashion. The quantitative application of this property in Los Angeles
was described by Bradley and Haagen-Smit[13].
Early investigators claimed that the cracking of
stretched rubber in the dark is a specific test for
ozone. However, more recently the Bell Telephone Laboratories at Murray Hill, New Jersey [14], have found that rubber is also cracked
by free radicals of many types in the manner
heretofore considered characteristic of ozone.
Some of their results have been obtained in the
absence of oxygen, precluding the possibility of
the formation of ozone by the free radicals. This
discovery of the effect of free radicals on rubber
has now deprived the investigator of any known
single specific chemical test for ozone. Other
"specific" reagents and methods mentioned in the
literature, such as "Tetrabase," the formation of
silver oxide, indigo sulfonic acid, and leuco-
fluorescein either lack sensitivity or have turned
out to be quite nonspecific.
Results
At Pasadena all the above described techniques have been used in combination with one
another. Table I shows the results of a comparison between the potassium-iodide and rubber-
cracking methods on two days. As can be seen,
the agreement between the two methods was
excellent; it was found to hold whenever the two
methods were used simultaneously. This appears
to rule out appreciable interference by nitrogen
dioxide in the iodide method at Pasadena, since
nitrogen dioxide does not give rubber cracking
typical of ozone. This evidence does not exclude
the possibility that some free radicals are present
that can oxidize iodide ion at a pH of 7.0 and
can also crack rubber.
In a further check, the method of Edgar and
Paneth [12] described above was tried. Considerable difficulty was encountered with the necessary drying train before the silica gel trap, presumably because of the reaction of ozone with
Table I.—Comparison between Iodide and Rubber-
Cracking Methods in the Pasadena Atmosphere
OZONE
PPHM
TIME OF DAY
IODIDE
RUBBER CRACKING
June 10,1952:
9:30 a.m.
8
8
10:30
9
7.5
11:30
13.5
12.5
11:50
20
22
12:30 p.m.
14
15
1:30
12.5
11
2:30
11
11.5
3:30
9
8
June 12,1952:
10:00 ajn.
12
11.5
11:00
15
11
11:10
17
18
12:15 p.m.
18
19
1:15
14
16
3:30
9
8
other constituents of the polluted atmosphere.
The calcium chloride used by Edgar and Paneth
was found to be quite unsuitable. This drying
problem was only partially solved, as may be
seen from the data reproduced in Table II.
For additional evidence, the gases absorbed on
the silica gel at liquid oxygen temperatures were
transferred to the optical cell of an ultraviolet
spectrograph by warming the trap to —80 °C. A
family of calibration curves had previously been
obtained for the particular combination of spectrograph and film used. The calibration curves
as well as the adsorption curve resulting from
this experiment are shown in Figure 13. It may
be observed that, at the same partial pressure,
the adsorption peak of the desorbed gas was not
quite at the same place (2630 A as compared
with 2610 A) as that of the calibrating, synthetic
Table II.—Comparison between Adsorption-
Desorption and Direct Iodide Determinations in the Pasadena Atmosphere
ozor»
E, PPHM
THROUGH
AFTER ADSORPTION
OUTSIDE AIR
CLE
AN-UP TRAINS
AND DESORPTION
37
21
16.6
22
6.9
14
11
5.0
11
3.6
0.7

32
THE SCIENTIFIC STORY
ji
dence to subsequent, less specific tests such as the
iodide method outlined above.
Another analytical method is based on the fact
that ozone is capable of cracking bent rubber
strips in a characteristic fashion. The quantitative application of this property in Los Angeles
was described by Bradley and Haagen-Smit[13].
Early investigators claimed that the cracking of
stretched rubber in the dark is a specific test for
ozone. However, more recently the Bell Telephone Laboratories at Murray Hill, New Jersey [14], have found that rubber is also cracked
by free radicals of many types in the manner
heretofore considered characteristic of ozone.
Some of their results have been obtained in the
absence of oxygen, precluding the possibility of
the formation of ozone by the free radicals. This
discovery of the effect of free radicals on rubber
has now deprived the investigator of any known
single specific chemical test for ozone. Other
"specific" reagents and methods mentioned in the
literature, such as "Tetrabase," the formation of
silver oxide, indigo sulfonic acid, and leuco-
fluorescein either lack sensitivity or have turned
out to be quite nonspecific.
Results
At Pasadena all the above described techniques have been used in combination with one
another. Table I shows the results of a comparison between the potassium-iodide and rubber-
cracking methods on two days. As can be seen,
the agreement between the two methods was
excellent; it was found to hold whenever the two
methods were used simultaneously. This appears
to rule out appreciable interference by nitrogen
dioxide in the iodide method at Pasadena, since
nitrogen dioxide does not give rubber cracking
typical of ozone. This evidence does not exclude
the possibility that some free radicals are present
that can oxidize iodide ion at a pH of 7.0 and
can also crack rubber.
In a further check, the method of Edgar and
Paneth [12] described above was tried. Considerable difficulty was encountered with the necessary drying train before the silica gel trap, presumably because of the reaction of ozone with
Table I.—Comparison between Iodide and Rubber-
Cracking Methods in the Pasadena Atmosphere
OZONE
PPHM
TIME OF DAY
IODIDE
RUBBER CRACKING
June 10,1952:
9:30 a.m.
8
8
10:30
9
7.5
11:30
13.5
12.5
11:50
20
22
12:30 p.m.
14
15
1:30
12.5
11
2:30
11
11.5
3:30
9
8
June 12,1952:
10:00 ajn.
12
11.5
11:00
15
11
11:10
17
18
12:15 p.m.
18
19
1:15
14
16
3:30
9
8
other constituents of the polluted atmosphere.
The calcium chloride used by Edgar and Paneth
was found to be quite unsuitable. This drying
problem was only partially solved, as may be
seen from the data reproduced in Table II.
For additional evidence, the gases absorbed on
the silica gel at liquid oxygen temperatures were
transferred to the optical cell of an ultraviolet
spectrograph by warming the trap to —80 °C. A
family of calibration curves had previously been
obtained for the particular combination of spectrograph and film used. The calibration curves
as well as the adsorption curve resulting from
this experiment are shown in Figure 13. It may
be observed that, at the same partial pressure,
the adsorption peak of the desorbed gas was not
quite at the same place (2630 A as compared
with 2610 A) as that of the calibrating, synthetic
Table II.—Comparison between Adsorption-
Desorption and Direct Iodide Determinations in the Pasadena Atmosphere
ozor»
E, PPHM
THROUGH
AFTER ADSORPTION
OUTSIDE AIR
CLE
AN-UP TRAINS
AND DESORPTION
37
21
16.6
22
6.9
14
11
5.0
11
3.6
0.7